1. Field of the Invention
The present invention relates to a speaker cone comprised of a novel material and to a process for the production of the same.
2. Description of the Related Art
As the material for conventional speaker cones, various materials have been used, such as natural wood pulp, synthetic pulp comprised of synthetic fibers or plastic, natural fibers, synthetic fibers, carbon fibers, glass fibers, or films of plastic (for example, polypropylene). Speaker cones are manufactured utilizing the characteristics of these materials.
Speaker cones using, as a raw material, natural wood pulp are produced by adding to a digested and beaten pulp various additives, such as fine carbon powder or resins, depending upon needs, to prepare a papermaking stock, forming this stock into paper in the shape of a speaker cone by a papermaking frame, then placing this in a speaker cone shaping mold, followed by applying heat and pressure.
This production process had the problems of a long, complicated process up to the preparation of the stock or raw materials. Also, since various additives, such as fine carbon powders and resins, were normally added, there was a remarkably detrimental effects on the work environment due to the splashing of these additives. Also, the waste water and solid waste including these additives are factors causing environmental pollution.
Also, as mentioned earlier, since the speaker cone is first made by a papermaking frame, there was a tendency for unevenness of thickness in the speaker cone thus shaped. In addition, there was a tendency for variation in the weights of the speaker cones produced. These posed a problem in quality control. How much the unevenness in thickness and the variation in weight can be reduced is an important point in the speaker conemaking process. At the present time, however, sufficient improvement has yet to be seen.
As one method of resolving the problem, the method may be considered of to first make the speaker cone stock and process it to a sheet shape in advance, then heat and press that sheet using a mold to form the speaker cone. In this method, however, since the speaker cone is three-dimensional, a drawing force acts on the sheet, but the sheet obtained by papermaking cannot sufficiently handle this drawing force and therefore the sheet will partially tear or stretch out of shape.
To give the sheet a strength able to handle the above-mentioned drawing force, it is preferable to use long fibers, but the length of the usable fiber when producing a sheet by the papermaking method is 1 to 5 mm. It is extremely difficult to make a uniform sheet by the papermaking method from a long fiber of over 5 mm length, in particular over 10 mm length.
Therefore, it is extremely difficult to form a speaker cone by the method of heating and pressing a sheet produced by papermaking using a mold as mentioned above.
When natural wood pulps are used as a raw material, the qualities thereof are varied based upon the origins of the woods and this causes the variances of the audio characteristics of the speaker cone obtained therefrom. In addition, even if the same natural wood is used, the audio characteristics are similarly varied depending upon the quality of water used for the production of pulps.
Furthermore, since the natural woods have high affinity with water, the speaker cone obtained therefrom is strongly influenced by humidity in using environments thereof, and therefore, the sound tone is remarkably changed. This influence provides remarkable limitation for the use of the speaker cone in, for example, outdoors or in the inside of automobiles.
Further, when the stock is a plastic, for example, polypropylene, the pellets of the plastic stock are shaped into a film, which is then vacuum molded. Thereafter, the bonding between the speaker cone body and the edge portions is improved by a plasma treatment, flame treatment, primary coating, or other processes.
In this way, when using a plastic such as polypropylene for the stock, the work up to the papermaking process is somewhat simplified over the case of using a natural wood material or synthetic pulp as the stock, but after shaping the speaker cone, complicated processes such as plasma treatment, flame treatment, or primary coating are necessary.
On the other hand, as a speaker cone produced by beating acrylic synthetic fibers to process them into a pulp and shaping that pulp, there are known a speaker cone comprised of a cone paper made by beating acrylic synthetic fibers having a large number of long and narrow voids inside them along the longitudinal direction to process them into a pulp, then making that pulp into paper (Japanese Unexamined Patent Publication No. 3-154600), a speaker cone comprised of a diaphragm formed by mixing and beating wood pulp and porous acrylic fibers and making the resultant mixed pulp into a paper (Japanese Unexamined Patent Publication No. 57-196694), etc.
Japanese Unexamined Patent Publication No. 3-154600 describes a speaker cone manufactured by papermaking the pulp comprising acrylic synthetic fibers having the special fiber structure, followed by forming the sheet into the speaker cone, there are fewer wrinkles occurring when forming it into a speaker cone compared with natural pulp, but in the papermaking process, due to the poor dispersability of the pulp in water, the occurrence of clumps of fibers is unavoidable and there are extremely large unevenness of thickness and variation of weight of the resultant speaker cones. Further, the pulp has a slower papermaking speed compared with natural pulp and is extremely low in productivity.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 57-196694 discloses a speaker cone comprising a mixture of a porous acrylic fiber pulp and a wood pulp, and the production process thereof, there are problems similar to the case when producing a speaker cone from a natural wood pulp.
Further, these conventional speaker cones have the defect that the internal loss (tan.delta.), one of the audio characteristics, is very susceptible to changes in the production conditions or the after-treatment, such as the resin coating of the speaker cone and the laminating of the speaker cone with a metallic foil. Further, when changing the audio characteristics by mixing in carbon fiber, an inorganic fiber, or an aromatic polyamide fiber or other high strength, high Young's modulus fiber, the internal loss changes considerably, so it was not possible to obtain a speaker cone having ideal audio characteristics.
Japanese Unexamined Patent Publication (Kokai) No. 2-175915 discloses acrylic synthetic fibers having a large number of long and narrow voids extending internally along the longitudinal direction and the production process thereof, Japanese Unexamined Patent Publication (Kokai) No. 2-200857 discloses a non-woven fabric comprising the above-mentioned fibers and the production method thereof.